1 Topic 10 Abstract Classes “I prefer Agassiz in the abstract, rather than in the concrete.”

1

Topic 10Abstract Classes

“I prefer Agassiz in the abstract,

rather than in the concrete.”

CS314 Abstract Classes 2

Back to the Property Example There are properties on a monopoly

board Railroads, Utilities, and Streets are

kinds of properties One behavior we want in Property is the

getRent method problem: How do I get the rent of

something that is “just a Property”?


The Property classpublic class Property {

private int cost; private String name;

public int getRent() {return hmmmmm?????

}

Doesn’t seem like we have enough information toget the rent if all we know is it is a Property.


Potential Solutions1. Just leave it for the sub classes.

Have each sub class define getRent()

2. Define getRent() in Property and simply return -1.

Sub classes override the method with more meaningful behavior.


Leave it to the Sub - Classes// no getRent() in Property

public void printRents(Property[] props) {for(Property p : props)

System.out.println(p.getRent());

}

Property[] props= new Property[2];props[0] = new Railroad("NP", 200, 1);props[1]

= new Utility("Electric", 150, false);printRents(props);

What is result of above code?A. 200150 B. different every timeC. Syntax error D. Class Cast ExceptionE. Null Pointer Exception


Fix by Casting// no getRent() in Propertypublic void printRents(Property[] props){ for(Property p : props) { if(p instanceof Railroad) System.out.println( ((Railroad)).getRent() );

else if(p instanceof Utility) System.out.println( ((Utility)p).getRent() );

}}Property[] props= new Property[2];props[0] = new Railroad("NP", 200, 1);props[1] = new Utility("Electric", 150, false);printRents( props);

What happens as we add more sub classes of Property?

What happens if one of the objects is just a Property?


Fix with Dummy Method// getRent() in Property returns -1

public void printRents(Property[] props) {for(Property p : props)

System.out.println(p.getRent());}

Property[] props= new Property[2];props[0] = new Railroad("NP", 200, 1);props[1] = new Utility("Electric", 150, false);printRents( props);

What happens if sub classes don’t override getRent()?

Is that a good answer?


A Better Fix We know we want to be able to find the rent

of objects that are instances of Property The problem is we don’t know how to do that

if all we know is it a Property Make getRent an abstract method Java keyword


Making getRent Abstractpublic class Property {

private int cost;private String name;

public abstract int getRent();// I know I want it. // Just don’t know how, yet…

}

Methods that are declared abstract have no bodyan undefined behavior.

All methods in a Java interface are abstract.


Problems with Abstract Methods

Given getRent() is now an abstract method what is wrong with the following code?

Property s = new Property();System.out.println(s.getRent());


Undefined Behavior = Bad Not good to have undefined behaviors If a class has 1 or more abstract methods,

the class must also be declared abstract. – version of Property shown would cause a

compile error

Even if a class has zero abstract methods a programmer can still choose to make it abstract– if it models some abstract thing– is there anything that is just a “Mammal”?


Abstract Classespublic abstract class Property { private int cost; private String name;

public abstract double getRent(); // I know I want it. // Just don’t know how, yet…

}// Other methods not shown

f a class is abstract the compiler will not allow constructors of that class to be calledProperty s = new Property(1, 2);//syntax error


Abstract Classes In other words you can’t create instances of

objects where the lowest or most specific class type is an abstract class

Prevents having an object with an undefined behavior

Why would you still want to have constructors in an abstract class?

Object variables of classes that are abstract types may still be declaredProperty s; //okay


Sub Classes of Abstract Classes Classes that extend an abstract class must

provided a working version of any abstract methods from the parent class – or they must be declared to be abstract as well– could still decide to keep a class abstract

regardless of status of abstract methods


Implementing getRent()

public class Railroad extends Property {

private static int[] rents = {25, 50, 10, 200};

private int numOtherRailroadsOwned;;

public double getRent() { return rents[numOtherRailroadsOwned];}

// other methods not shown}


A Utility Class


Polymorphism in Action// getRent() in Property is abstract

public void printRents(Property[] props) { for(Property p : props)

System.out.println(p.getRent());}

•Add the Street class. What needs to change in printRents method?•Inheritance is can be described as new code using old code. •Polymorphism can be described as old code using new code.


Comparable in Propertypublic abstract class Property

implements Comparable<Property> {

private int cost; private String name;

public abstract int getRent();

public int compareTo(Property other) { return this.getRent()

– otherProperty.getRent(); }}

Back to Lists We suggested having a list interface

public interface IList<E> extends Iterable<E> {

public void add(E value);

public int size();

public E get(int location);public E remove(int location);

public boolean contains(E value);

public void addAll(List<E> other);public boolean containsAll(List<E> other);

}


Data StructuresWhen implementing data structures:

-Specify an interface

-Create an abstract class that is skeletal implementation interface

-Create classes that extend the skeletal interface


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1 Topic 10 Abstract Classes “I prefer Agassiz in the abstract, rather than in the concrete.”